Spacecraft's photos confirm volcanism on planet Mercury

On its second flyby of the planet Mercury last October, NASA's Messenger spacecraft beamed back 1,200 pictures, revealing 30 percent of the planet's surface that had never been seen up close before.

Among the images was a view of the second-largest crater on Mercury, almost as wide as the distance from San Diego to San Francisco.

Called the Rembrandt basin – by convention, Mercurian craters are named after painters, authors, composers and other creative artists – the crater is surprising more for the geological features within its rim than for its size, planetary scientists say.

Much of the terrain inside Rembrandt remains pristine from the time of impact, about 3.9 billion years ago, near the end of the “heavy bombardment” period of the early solar system. The basin is young compared with most of the giant, ancient impact craters in the solar system.

“In most large impact basins on Mercury, the moon and the other inner planets, this terrain is completely buried by volcanic flows erupted after the basin formed,” said Thomas R. Watters, a senior scientist at the Smithsonian Institution's Center for Earth and Planetary Studies.

In Rembrandt's central region where lava did flow, the terrain has deformed into ridges and troughs that radiate outward in a wheel-and-spoke pattern – “unlike any we have seen in any other impact basin in the solar system,” Watters said.

Ridges form when planetary crust is pressed together, troughs when the crust is pulled apart. Therefore, scientists will have to devise an explanation of how the crust within the Rembrandt basin could have been both pressed together and pulled apart to form adjacent ridges and troughs.

Another remarkable feature is a line of cliffs 620 miles long that cuts into the Rembrandt crater. Mercury's surface cracked as its interior cooled and shrank, and this fault – perhaps the longest of these cracks – shows that Mercury was still shrinking at the time of the impact.

More globally, Messenger's images indicate that smooth plains, most probably consisting of hardened lava flows, make up 40 percent of the surface, compared with 20 percent on the Earth's moon.

“This indicates that volcanism was a really important process on Mercury,” said Brett Denevi, a postdoctoral researcher at Arizona State University. “Which is pretty exciting, because before Messenger's flybys of Mercury, just a year and a half or so ago, we were really not even sure that volcanism existed on Mercury.”

She said that comparison of different terrains suggested that although Mercury was similar to the moon in appearance, its geological history may have been more like that of Mars.

In articles this year, scientists reported the first detection of magnesium in Mercury's tenuous atmosphere by a Messenger instrument that samples its surroundings. Because much of Mercury's “air” consists of molecules knocked off the surface, this discovery helps confirm the presence of magnesium in the crust, which is not a surprise.

“What is surprising is the distribution of magnesium,” said William McClintock of the University of Colorado's Laboratory for Atmospheric and Space Physics and a co-investigator of the mission.

Hydrogen, helium, sodium, calcium and potassium had been detected before. Calcium and magnesium are similar chemically, and the distribution of the two elements was expected to be similar. Instead, calcium peaked near the equator, while the distribution of magnesium was more uniform.

Messenger will make one more flyby of Mercury, on Sept. 29, before entering orbit around the planet in March 2011.